1. Field of the Invention
The present invention generally relates to ultrasonic inspection methods and systems. More particularly, this invention relates to a method and system for ultrasonically inspecting a disk-shaped article having a central opening and a plurality of circumferentially-spaced secondary openings that are radially spaced outward from the central opening, such as a turbine wheel or disk, wherein ultrasonic signals are generated and received by transducers placed in the secondary openings.
2. Description of the Related Art
Ultrasonic inspection techniques have been widely used to perform nondestructive testing on articles formed of materials having an intrinsically coarse grain structure, which results in anisotropic and nonuniform acoustic properties. Particular examples of such articles include forged super-alloy turbine wheels (rotors) used in gas and steam turbines. In the hostile operating environments of gas and steam turbines, the structural integrity of the turbine wheel is of great importance in view of the high mechanical stresses that wheels must be able to continuously withstand at extremely high temperatures.
Ultrasonic inspection techniques employed with turbine wheels have typically involved inspecting each forging from a plane perpendicular to the highest operating stresses, i.e., ultrasonic transducers are placed on the fore and/or aft surfaces transverse to the rotational axis of a turbine wheel. With this approach, the ultrasonic energy is generated in a direction substantially perpendicular to the orientation of the most common defects, which tend to lie in axial-radial planes of a turbine wheel. Two ultrasonic testing techniques have been conventionally used, a first of which is a pitch-catch technique using two transducers placed on the fore and aft surfaces of the wheel. One of the transducers serves to generate an ultrasonic signal, while ultrasonic signals reflected from acoustical discontinuities being received by the second transducer. The second common technique is referred to as pulse-echo and makes use of a single transducer to both generate the ultrasonic signal and receive reflected signals.
Inspections targeting axial-radial defects are performed routinely on steam turbine discs with shrunk-on wheels. Stress corrosion cracking typically develops at the surface of the central bore, and such cracks can be located using the pitch-catch technique in which the ultrasonic signals are targeted at the bore and the keyway surfaces at the perimeter of the wheel. This technique is not intended for full volumetric inspection of the wheel. A nondestructive testing method using the pitch-catch technique is also disclosed in U.S. Pat. No. 5,189,915 to Reinhart et al., whereby wheel blade attachment locations can be inspected ultrasonically from the bore of the wheel. Another pitch-catch technique has been developed for gas turbine wheels in the as-forged condition, at which time the wheels have parallel sides so that the technique is relatively simple to execute.
A pitch-catch inspection technique using transducers placed on the fore and aft faces of a gas turbine wheel following final machining would also seem the most likely technique to detect flaws having an axial-radial orientation throughout the wheel volume. However, axial-radial defects are difficult to detect in gas turbine wheels due to their complex geometry resulting from the final machining operation, which includes the machining of circumferentially-spaced bolt holes spaced radially from the central bore. A pitch-catch inspection of such a wheel would require a complex movement of the transducers and control of transducer angles to ensure an effective examination. Such an approach would be further exacerbated by the high sonic noise produced by the large material grain size as well as a tendency for the acoustic pulse to be steered by flow lines produced during the forging process.
In view of the above, it would be desirable if an improved method were available for ultrasonically inspecting articles having complex geometries, such as gas turbine wheels following final machining.